This invention relates generally to an implantable sensor with wireless communication, and particularly, but not by way of limitation, to physiological monitoring of pressure or other parameters in humans and animals using a monitor that is implantable within a heart chamber or elsewhere and is capable of wireless communication of sensor information therefrom.
The monitoring of fluid pressure within a body organ provides an important tool for medical research and clinical diagnosis. For example, hydrocephalus and head injuries can cause body fluids to build up within the brain. The resulting fluid pressure buildup can result in death or serious brain damage. In another example, urinary dysfunction can cause fluid pressure to build up in the bladder. In a further example, intrapleural pressure measurements can be used to monitor the respiration of infants who have been identified as being at risk for sudden infant death syndrome.
Blood pressure measurements are particularly important for medical research and diagnosis for a variety of reasons. Such measurements provide researchers with insight into the physiology and functioning of the heart. Blood pressure measurements also provide researchers with useful information regarding the safety and efficacy of pharmaceuticals and the toxicity of chemicals. By transducing blood pressure into a signal waveform, a variety of useful parameters can be extracted. These parameters provide valuable information for the diagnosis of heart disease. Left ventricular (LV) blood pressures measurements are particularly important because the left ventricle chamber of the heart pumps blood to the systemic circulatory system, that is, throughout the rest of the body.
Common parameters extracted from left ventricular blood pressure waveforms include peak systolic pressure (the high pressure peak resulting from a contraction of the left ventricle chamber of the heart), end diastolic pressure (the low pressure valley resulting from expansion of the left ventricle), and maximum dP/dt (a peak value of how fast the pressure (P) changes with time (t) during a contraction of the left ventricle). These blood pressure measurements provide helpful diagnostic information to the physician.
For example, maximum dP/dt provides a measure of the work that is being done by the heart. For certain conditions, such as congestive heart failure (CHF), it is desired to reduce the work load on the heart. The treating physician can determine how effective a therapy is by determining if the treatment regimen has indeed reduced the work load on the heart, as indicated by the maximum dP/dt signal extracted from the left ventricular blood pressure waveform. Measurement of left ventricular blood pressure is also useful for titrating new drugs for treating heart disease, that is, determining the desired dosage or concentration of a new drug. Titrating new drugs requires information on how these drugs are affecting the heart.
For example, beta adrenergic blocking drugs are often effective at treating arrhythmias and improving patient hemodynamics. However, such drugs are difficult to titrate. Because left ventricular blood pressure parameters, such as maximum dP/dt, provide information on how the heart is functioning, monitoring these parameters allows a physician to more easily determine the most appropriate dose of the drug for treating the patient. The maximum dP/dt signal, if available, could also be used as a feedback mechanism in a system that automatically delivers therapy to adjust the work load of the heart. The delivery of therapy is automatically adjusted based on the work load of the heart, as indicated by the maximum dP/dt signal.
In another example, left ventricular blood pressure provides useful information for controlling a cardiac rhythm management system. Cardiac rhythm management systems include, among other things, pacemakers, or pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart. Heart contractions are initiated in response to such pace pulses. By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly. Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn""t allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other systems or devices for diagnosing or treating cardiac arrhythmias.
One example of using a cardiac rhythm management device to control heart rate in proportion to left ventricular blood pressure is described in Mehra U.S. Pat. No. 5,129,394. The ""394 patent, however, does not disclose sensing actual left ventricular blood pressure. Instead, it discloses a pressure sensor located in the coronary vein. The coronary vein extends from the right atrium through the heart tissue near the left ventricle. Because of its small size, the coronary vein is difficult to access for inserting a pressure sensor. Moreover, blood pressure sensing in the coronary vein provides only an indirect approximation of the actual left ventricular blood pressure.
Other existing techniques for monitoring left ventricular blood pressure also have drawbacks. One technique of measuring left ventricular blood pressure is described in Brockway et al. U.S. Pat. No. 4,846,191, which is assigned to the assignee of the present application. The ""191 patent describes a pressure sensor that is implanted in the abdomen of a laboratory animal. The pressure sensor is connected to an organ, such as the heart or the brain, via a fluid-filled pressure transmitting catheter (PTC). One limitation of this device is that it requires invasive access to the organ in which fluid pressure is to be monitored.
For example, in monitoring left ventricular pressure, one surgical technique for using the device described in the ""191 patent involves performing a highly invasive laparotomy procedure. In this procedure, the pressure transmitting catheter is passed through an incision in the diaphragm and an incision into the apex (bottom tip) of the heart. The high blood pressure in the left ventricle further increases the risk of making such incisions directly into the left ventricle. This procedure typically requires a two week recovery period for the laboratory animal. Moreover, because catheterization of the apex involves significant risks, this technique would likely be considered too invasive for human use.
Alternatively, an incision may be made into the aorta, which is the primary artery carrying blood from the left ventricle to the rest of the body. The pressure transmitting catheter is then passed into the aortic incision for measuring blood pressure in the aorta. Aortic incisions are also problematic because of the high blood pressure in the arterial circulatory system. Moreover, measuring blood pressure in the aorta does not provide a direct measurement of blood pressure in the left ventricle; such information is unavailable, for example, when the aortic valve is closed. Alternatively, the pressure transmitting catheter could be passed through the aortic valve into the left ventricle. However, leaving the pressure transmitting catheter extending through the aortic valve for a long period of time risks damage to the aortic valve as a result of the high blood pressure in the left ventricle. Thus, this procedure is also likely unsuitable for human use, particularly for chronic left ventricular blood pressure monitoring, i.e., monitoring over an extended period of time.
Another technique for measuring left ventricular blood pressure is described in Pohndorf et al. U.S. Pat. No. 5,353,800. A distal end of a pressure sensing lead is transvenously introduced into the right ventricle of the patient""s heart. A hollow needle at the distal end of the lead is punched through the ventricular septum, that is, through the wall separating the right and left ventricles. This provides access to the left ventricle for sensing pressure gradients that are communicated through the hollow needle to a pressure sensor that is outside of the left ventricle. Because this procedure involves invasively forming an opening in the septum, it creates significant risks for human cardiac patients who are likely already very sick and vulnerable to such risks.
A further technique for measuring left ventricular blood pressure uses a pressure sensing catheter, such as a xe2x80x9cMillar catheter,xe2x80x9d available from Millar Instruments, Inc., of Houston, Tex. The pressure sensing catheter is passed through the left atrium and through the mitral valve (which separates the left atrium and left ventricle) into the left ventricle. As discussed above, however, high blood pressures exist in the left ventricle, which would likely result in damage to the mitral valve if the catheter were left interposed in the mitral valve for a long period of time. As a result, if a sequence of successive measurements is to be obtained over a long period of time, the patient must undergo recatheterization for each measurement. However, catheterization itself involves risk, discomfort, and expense, making multiple catheterizations of the patient very undesirable.
In summary, present techniques for measuring left ventricular pressure are too invasive for human use and unsuitable for use over an extended period of time. Physicians and researchers need less invasive techniques for chronic measurement of left ventricular blood pressure, both for diagnosing heart conditions and for determining whether therapy delivered to the heart is adequate for effectively treating the patient""s symptoms.
The present system provides, among other things, a less invasive implantable sensor device capable of wirelessly communicating sensor information. The sensor is implantable in a heart chamber, in other body organs and body cavities, and elsewhere within a living organism. One example includes a blood pressure monitoring device that is suitable for use over an extended period of time in the left ventricle for wirelessly communicating blood pressure information therefrom. This provides less invasive chronic pressure measurements in the left ventricle. As a result, the risk of obtaining such important measurements is reduced. This enables a physician to more accurately diagnose and treat serious heart conditions. It also enables a biomedical researcher to monitor sensor signals in animal research studies.
In one example, the wirelessly communicated left ventricular blood pressure information is used to control the delivery of therapy by a cardiac rhythm management device. In another example, the present system advantageously allows a physician to obtain a sequence of left ventricular blood pressure measurements over a long period of time. By contrast, using a pressure sensing catheter for obtaining such measurements over a long period of time risks damaging heart valves because of the high blood pressures that exist in the left ventricle. Because the present system allows long term monitoring, it can be used, for example, in assessing circadian variations in physiological data over a period of time. Such information is potentially valuable in diagnosing and treating patients. See, e.g., Brian P. Brockway, Perry A. Mills, and Sylvia H. Azar, xe2x80x9cA New Method For Continuous Chronic Measurement and Recording of Blood Pressure, Heart Rate, and Activity in the Rat via Radio-Telemetry,xe2x80x9d Clinical and Experimental Hypertensionxe2x80x94Theory and Practice, A13(5), pp. 885-895 (1991), which is incorporated herein by reference in its entirety.
Certain particular embodiments of the invention are summarized below, by way of illustrative example, but not by way of limitation. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.
One aspect of the invention provides an apparatus for sensing a parameter in a heart chamber in a heart in a living organism. The apparatus includes a sensor and a wireless communication circuit. The sensor is adapted for being disposed in the heart chamber. The sensor provides a sensor signal based on the parameter sensed in the heart chamber. The wireless communication circuit is adapted for being disposed in the heart chamber. The communication circuit is coupled to the sensor and transmits information out of the heart chamber based on the sensor signal. The wireless communication techniques include radio-telemetry, reactive coupling, passive transponders, and intracorporeal conductive communication.
In one embodiment, the sensing apparatus includes a housing carrying the sensor and the communication circuit and at least one stabilizer that is coupled to the housing. Also included in the housing is a battery which, in one embodiment, is recharged by energy received from outside the heart chamber. A receiver, external to the heart chamber, is communicatively coupled to the communication circuit for receiving the information based on the sensor signal. In one embodiment, the receiver is carried by a cardiac rhythm management system, and therapy delivered by the cardiac rhythm management system is adjusted according to information wirelessly received from the sensor device implanted in the heart chamber. In another embodiment, the receiver is coupled to a computer that analyzes or displays the information from the sensor. In one embodiment, the sensor is a pressure transducer, however, other sensors may also be used.
Another aspect of the invention includes a method of sensing a parameter (e.g., blood pressure) in a heart chamber in a heart in a living organism. A physical manifestation of the parameter in the heart chamber is received at a sensor disposed within the heart chamber, where it is transduced into a sensor signal. Information based on the sensor signal is wirelessly communicated from the heart chamber. A further embodiment includes translumenally disposing the sensor in the heart chamber.
One embodiment of communicating the information includes using a passive transponder. In this technique, energy is received from outside the heart at a passive transponder that is in the heart. The passive transponder is powered from the energy received from outside the heart chamber. Information is transmitted from the heart chamber using the powered passive transponder. In another embodiment, energy received from outside the heart chamber is used to recharge a battery that is located in the heart chamber.
Another embodiment of communicating information includes using intracorporeal conductive communication, which uses the living organism as the conductor. In this technique, a current is conducted through at least a portion of the living organism. A signal that is based on this current is received at a receiver that is outside the heart chamber. In one embodiment, the receiver is carried by an implantable medical device located within the living organism such as, for example, a cardiac rhythm management device. Therapy delivered by the cardiac rhythm management device is adjusted based on the signal received by intracorporeal conductive communication or other wireless communication technique. In another embodiment, the receiver is external to the living organism, and information is stored in a memory in the receiver.
Another aspect of the invention provides a method. The method includes inducing a current between first electrodes implanted in a living organism. The current at the first electrodes is modulated with a data signal. A signal based on the current is demodulated at second electrodes. In one embodiment, the second electrodes are also implanted in the living organism.
Another aspect of the invention provides a catheter. The catheter includes an elongate member having first and second ends. The first end of the elongate member includes a cavity adapted for carrying an implantable measurement device that includes a wireless communication circuit. The elongate member also includes a lumen extending substantially between the cavity and the second end of the elongate member. An engaging member is carried by the cavity. The engaging member is extendable outwardly from the cavity at the first end of the elongate member. The engaging member is operatively coupled to a manipulator at the second end of the elongate member. The engaging member is adapted for engaging the implantable measurement device. In one embodiment, portions of the elongate member are flexible such that the catheter is adapted for translumenal access to a heart chamber. Other aspects of the invention will be apparent on reading the following detailed description of the invention and viewing the drawings that form a part thereof.